1. Field of the Invention
The present invention relates to a process for fabricating a compound semiconductor device, and more particularly, to a process which comprises epitaxially growing a compound semiconductor layer containing at least arsenic on the surface of a single crystal silicon substrate.
2. Description of the Related Arts
It has been believed difficult to grow a high-quality single GaAs crystal on the surface of a silicon substrate due to the differences between those two substances in lattice constants and in thermal expansion coefficients. However, a high-quality single GaAs crystals can be grown directly on a silicon substrate by employing MOCVD (metallorganic chemical vapor deposition) techniques, provided that specified conditions are selected, as disclosed in JP-A-Sho-63-133616 (the term "JP-A-" referred herein signifies "an unexamined published Japanese patent application"), JP-B-Hei-2-36059 (the term "JP-B-" as referred herein signifies "an examined published Japanese patent application"), JP-B-Hei-2-36060, etc.
It has been revealed, however, that the GaAs layer obtained by the aforementioned process contains, as shown in FIG. 13, the carrier distribution profile along the depth direction of a GaAs layer on a silicon substrate, silicon as an impurity at a density as high as to amount to about 10.sup.17 cm.sup.-3. Accordingly, an n-GaAs/p-GaAs/undoped GaAs structure fabricated by this process fails to have its carrier density controlled due to the presence of silicon impurities. In FIG. 13, the characteristic curve indicated by L1 is for the density of the silicon impurity measured with a secondary ion mass spectrometer (SIMS), and the characteristic curve L2 represents the density of active silicon (carrier density) as obtained by a C-V method using a Profile Plotter (a product of Polaron Corp.).
This phenomena of introducing silicon impurities into a MOCVD-deposited GaAs layer on a silicon substrate has been pointed out also by S. Nozaki et al., in Appl. Phys. Lett., 57(25), p. 2669 (Dec. 17, 1990), but the mechanism of taking up silicon impurities is still unclarified.